Abstract:

Solid materials containing frustrated Lewis pairs (FLPs) as active sites have attracted much attention due to their ability to activate and transform small molecules. However, it is still highly challenging to precisely construct FLP sites on the surfaces of nanomaterials, thereby limiting the applications of these materials. Nanostructured ceria (CeO₂) is commonly employed as a catalyst or functional support, and exhibits both Lewis acid and basic properties as well as abundant and easily regulated surface defects, which originate from the reversible Ce³⁺/Ce⁴⁺ redox pair. When the Lewis acid and base sites of CeO₂ are independent of each other, the combined Lewis acid-base sites play a similar role to that of homogeneous FLP sites. Thus, the rich surface properties of nanostructured CeO₂ provide significant potential for the construction of solid FLPs.

Herein, we demonstrate that solid FLP sites can be successfully constructed on the surface of CeO₂(110) via the regulation of surface defect clusters, which can be used to create new Lewis acid sites composed of two adjacent Ce³⁺ atoms on the surface. Novel interfacial FLP sites can then be formed by combining these Lewis acid sites with neighboring surface lattice oxygens, which act as Lewis base sites. Porous CeO₂ nanorods (PN-CeO₂) with boundary surface defects were prepared by a special two-step hydrothermal process, and exhibited remarkable catalytic FLP properties. Hydrogen molecules could be effectively activated on the surface of PN-CeO₂ with a low activation energy of 0.17 eV via a heterolytic cleavage process. Hydrogenation of alkenes and alkynes to alkanes could then be realized by the activated hydrogen under mild reaction conditions.

PN-CeO₂ nanorods with FLP active sites were also able to activate CO₂ molecules effectively. Unlike in other solid FLP sites, CO₂ molecule activation was realized via a Lewis base site binding with the C atom while two Lewis acid sites bound the two O atoms, owing to the unique configuration of the FLP sites in PN-CeO₂. When combined with the epoxidation of olefins by "isolated" Ce³⁺ sites in PN-CeO₂, the FLP-inspired activated CO₂ could be used to transform olefins and CO₂ to cyclic carbonates through a selective tandem transformation route. In addition, density functional theory studies indicate that the FLP sites on CeO₂(110) can activate the C―H bond of CH₄ with activation energies as low as 0.63 eV, which can be attributed to the enhanced acidity and basicity of the FLP sites.

With this improved understanding of solid FLP sites constructed on ceria, we have also been able to summarize the challenges and prospects in this field, including their construction, characterization, and mechanism analysis.

Key words: Frustrated Lewis pairs, CeO₂, Surface defect, H₂ activation, CO₂ activation, CH₄ activation